The className of the HTMLElement containing the TabView's label elements
to look for when building from existing markup, or to add when building
from scratch.
All childNodes of the content container are treated as content elements when
building from existing markup.

The className of the HTMLElement containing the TabView's tab elements
to look for when building from existing markup, or to add when building
from scratch.
All childNodes of the tab container are treated as Tabs when building
from existing markup.